Getting the Rear Right

I mentioned the shocks above in the tuning guide but didn’t provide any guidance because this “how-to” started life as “fork-centric” and I hadn’t found a solution that I liked for my bike. Having done the necessary research for my bike I thought I’d share the knowledge.

As mentioned above, the forks and shocks are a system and need to work in harmony. For example, let’s say you follow every bit of advice given above and get your forks perfectly in the sweet-spot (good for you). If your shocks are too soft or under-dampened your forks will be underloaded and won’t be able to move through their full stroke. Further, soft shocks effectively increase steering rake, making the steering light and vague.

If your shocks are too stiff you will overburden the forks, causing you to dial-in an overcompensating amount of rebound during tuning. Remember, rebound is the intersection between ride and handling. Too much rebound and the forks will feel darty/twitchy.

But most importantly, shocks define how comfortable the bike will ride. So it’s important for performance and comfort to get the shocks right.

What makes one shock better then another? Like the forks, the primary function of the shock is to 1) support the sprung weight of the bike and rider and 2) provide damping to control the speed of movement throughout the stroke of the shaft. Without damping a spring will seek its static position as quickly as possible, overshooting its static position due to inertia. Also, just like the forks, a shock uses oil and air to create compression and rebound  damping. As the shock moves it creates heat which is absorbed into the oil. When the oil gets hot it foams within the chamber and if the oil gets too hot it causes cavitation (slipping) within the chamber – impacting the ability of the shock to provide predictable (or meaningful) damping. The faster that heat can be absorbed and dissipated, the better the shock can perform its primary function of providing predictable damping. As a rule: the bigger the oil chamber the more effective the heat management of the shock. Also, better shocks tend to have external adjustments for compression and rebound damping. This allows the rider/operator to dial-in (fine tune) the shock without having to remove the shock and make internal damping adjustments via the valving stack.

Let’s review the various types of shocks that are “reasonably” available for bikes like ours. Shocks come in three basic styles; Twin-Tube, Gas-Emulsion and Reservoir (piggyback reservoir & remote reservoir).

Twin-Tube shocks are the type most typically seen on budget bikes. They are self-contained with an inner oil chamber and an outer air chamber (hence, the twin tubes). The damping rod (shaft) is connected to a piston that moves through the oil, creating damping. The vast majority of these shocks are typically low performance, non-rebuildable, provide fixed damping and only adjustable for spring preload (usually just 5 positions for spring preload). Advantages – usually cheap ($50 to $200 pair). Disadvantages – limited adjustability, fixed dampening, small oil chamber and non-rebuildable, so when the damping no longer meets your needs you throw them away. Best use: play/monkey bikes, around town bikes.

Emulsion shocks (sometimes called gas-pressurized) are next on the shock hierarchy and offer a significant improvement over twin-tube shocks. Emulsion shocks are single tube shock that have a self-contained reservoir that separates the oil and air. Because of this they are not quite full of oil because they require an air space to deal with the displacement of the shock shaft. The better emulsion shocks use nitrogen instead of air and therefore are called “gas shocks”. Nitrogen is desirable over air because it doesn’t expand/contract with temperature changes like air. This is a desirable attribute since the shocks have a small gas/air chamber. The gas is compressed (pressurized gas) to raise the temperature and lower the pressure that cavitation occurs within the chamber. Once cavitation begins, the shock no longer provides meaningful damping. Advantages – not terribly expensive ($200 to $600 pair depending on features), typically rebuildable (including the damping stack/valving so the high-speed compression can be customized for your bike and riding style), typically infinite adjustability within a range for spring preload and on higher spec units adjustability for low-speed compression. Disadvantages – relatively small oil chamber so they don’t manage heat well. Best use: street bikes where the rider enjoys a comfortable ride and doesn’t do much “spirited” riding.

Up next are gas-pressurized reservoir shocks, both piggyback and remote. The difference between these two types of reservoir shocks is the size of the oil chamber and their ability to absorb and dissipate heat. Since the remote reservoir is detached from the shocks, it can more effectively manage heat (on higher-end units the valving is also in the remote). Because of this defining feature these types of shocks are usually considered “high performance”. Otherwise everything mentioned about gas-emulsion shocks applies to this type of shock. Advantages – large oil/gas chamber so they manage heat extremely well, typically rebuildable (including the damping stack/valving so they can be customized for your bike and riding style), typically infinite adjustability within a range for spring preload and on higher spec units adjustability for low-speed compression, high-speed compression and rebound. Disadvantages – expensive ($400 to $1,200 pair depending on features). Best use: track bikes.

The above is generalized but is a good primer and provides a basic knowledge of shocks.

The shocks found on the vast majority of budget bikes are of the twin-tube variety. So it’s no surprise that Suzuki uses twin-tube shocks on the LS650/S40. RYCA also supplies twin-tube shocks in their kit that appear to be the same quality as the OEM shocks. Unfortunately the shocks supplied by RYCA are under dampened (a design limitation) and to compensate for the poor motion control, they are also over sprung – the worst of both worlds. But if we’re being honest, if you’re riding with the RYCA fork solution, you’ll probably never notice the crappy shocks. If you have taken the time and spent the money to improve your front suspension, by now you are probably very unhappy with the shocks.

So if we are going to improve the shocks to work as well as the forks, what do we need/want?

First consideration is shock length. Turns out the RYCA guys got this spec spot-on. The kit shocks are 13” (330mm) eye-to-eye (from the center of the upper mounting hole to the center of the lower mounting hole) and can’t really be much longer. I tried a length of 13-3/8” (340mm) and experienced some chain contact at the front end of the swing arm during braking and bumpy road surfaces. It’s barely noticeable but I went back to 330mm and the contact was gone (335mm is probably a safe length). I wanted to experiment with the longer length to see if a rake of 26-27 degrees would improve the handling, but I found the steering to be a bit light for my liking. I could have made adjustments to the forks to compensate, but with the chain contact it wasn’t worth any of my effort. 13” works perfect.

Another consideration is spring rate. A RYCA conversion is a light bike with most of the weight bias towards the rear end. My bike weighs 330 lbs with fuel (157 front / 173 rear) without me and about 500 lbs with me (222 front / 278 rear). Since we don’t have a link suspension, we want progressively wound or dual-rate springs.

We need shocks with 14mm mounting eyelets.

We need shocks that provide more than 2 inches of travel. The short stroke is a result of cramming everything into one tube (oil, air chamber, valving, piston, rebound spring). This is also another reason why a reservoir shocks are desirable – they inherently provide more stroke.

So with the above information I decided to identify shocks for my café racer with the following criteria:

-      Adjustable length that allowed for lengths in the range of 330 to 340mm.
-      A reservoir type shock for the extra performance and additional stroke/travel.
-      A fully rebuildable shock so the valving can be suited to the attributes of a light-weight café racer.
-      External adjustments for compression and rebound damping would be “nice-to-haves” but not necessary because once the valving is correct, I typically don’t make further adjustments (just like our forks).
-      Here was the hardest one, I didn’t want to spend more than $500 for the pair.

There are many choices for shocks above $500 (closer to $1,000) from Fox (Podium), Ohlins (36P), RaceTech (G3-S), YSS (G-302) and Progressive (970 Series) to name a few. And then there are knock-offs of some of the above shocks that are typically below $100. I didn’t want to spend $700 to $1,200 for the originals and didn’t want to bother with the knock-offs.

There are always questions and skepticism about the quality of the knock-off shocks. In forums these questions usually quickly dissolve into arguments between people who have never used them who claim they are junk to people who own them and say they work. I’ve seen every claim on the junk side including they are made of pot metal and will disintegrate to there are no actual components (valving or piston) inside the shocks. On the surface this could be believable, and in some cases maybe even true. But there are a few knock-off manufacturers that seem to get good reviews form people who actually have used them, namely RFY and TEC.

The RFY and TEC reservoir shocks look a lot like a the Ohlins 36P or HO-142 shock (not a high-end racebike unit with external damping adjustments, but a good streetbike version that is fully rebuilable/adjustable) but they are a fraction the cost. At that price point can they actually be a viable option? On several of the places I was doing research (CB350 and Triumph forums) there was a gentlemen by the name of Chris that actually had experience rebuilding shocks and decided to do a tear-down and conduct a comparison of RFY shocks to Ohlin shocks. His findings were shocking (get it?). He found that the RFY copies are quite faithful to the originals. Moreover, he felt that the only thing that would keep him from recommending these shocks as an inexpensive streetbike shock was its set-up. Over time he developed a good working understanding of the RFY and TEC shocks and began to offer a rebuild service, as well as a custom build-to-order service of these shocks. It was the custom build-to-order service that caught my attention because if I decided to try this inexpensive alternative I knew they wouldn’t work on my bike out of the box. Further, I also do not possess the proper specialized tools to conduct a proper rebuild of a pair of shocks. Finding someone that could get the shocks right the first time took the risk out of the equation. So I provided my personal needs and my bike needs to Chris, and for a little over $200 I had shocks that worked out of the box for my bike.

These custom/rebuilt RFY shocks checked all of my boxes:

-      Adjustable length that allowed for lengths in the range of 330 to 340mm.
-      A gas-pressurized reservoir type shock for the extra performance and additional stroke/travel of 2.75”, an improvement of ¾ inch.
-      A fully rebuildable shock so the valving has been built to the attributes of my café racer.
-      14mm top mount and eyelets.

I hate to sound like a fanboy, but the shocks completely changed my enjoyment of my bike. Simply put, these shocks work. My rear suspension now matches the performance and comfort of my forks. The suspension is finally in balance.

Thanks for the in-depth write up, Gary!
When you refer to 2"  of travel or 2 3/4" of travel, are you referring to the actual amount the shock compresses, or to wheel travel?
The shocks are set inboard of the rear axle, so 2" of shock movement would translate into more than 2" of wheel travel.
One of the things that always made me crazy about Harleys is that a lot of them have about an inch of shock travel and 2" of rear wheel travel  
thanks,
-Armen

Thanks Gary.
Have you calculated rear wheel travel with the new shocks?
I'm ordering a set this week.
Thanks for the well written article and the lead!

When I get mine, I'll yank the springs, mount them, and measure the wheel travel.
All that thinking makes my brain hurt

Are you running an 18" or 19" front wheel?
Still have the 19" on mine. Prob end up with an 18"
thanks

Yup. My thinking as well. For now, I'm just trying to get the bike on the road. The rear wheel ended up costing about $600 with all the bells and whistles, so I have to pace myself a bit.
The 18" will also improve braking a tiny amount.

Dave,
Yup, got that. I ride a lot of customer bikes with 'full figured' front tires, and try to explain to the owners that the bike handles worse with wider tires.

Hey Gary,
Thanks so much for taking the time to do all that work, document it, and post it.
I'm ordering a set of shocks this week. Was going to last week, but ended up replacing my furnace, so I'm in a bit of sticker shock
I'm running a belt on mine. With the 2" travel RYCA shocks, the belt goes from crazy loose to pretty tight during it's travel. Wondering if the additional .75" of shock travel will make the belt tension even crazier.
I'll dummy it all up when I get the shock.
Worst case scenario, I'll make spacers to go between the shock bumpers and the top shock mount, and kill a bit of travel to reduce the range of belt slack. And still have a sexier shock in terms of damping.
If my math is correct, the wheel moves about 1.545" for every 1" the shock moves. Reducing the shock travel by .250" will still give 3.86" of wheel travel, which is prob enough for me. And more than the 3.09" afforded by the RYCA shock.
More stuff to do in my abundant spare time...